Electric characteristics of a silicon wafer as a starting material are altered by impurity elements included in the silicon wafer and attached thereto during heat treatment of a device fabrication process, which cause device failure; therefore, to avoid such an adverse influence due to impurities has been more and more important with the advent of highly sophisticated devices and become a great problem to be solved in the device fabrication as well.
As one measure against the adverse influence, a method has been put into practice in which a polycrystal silicon layer (PBS, Poly Back Seal, film) 62 is formed on a back surface of a silicon wafer W as shown in FIG. 10 as well as grain boundaries present in the PBS film are utilized as an absorption source for the impurities, and a wafer and an epitaxial wafer both with a so-called PBS film on a back surface thereof have been utilized as products.
Especially, boron among the IIIB group elements is positively utilized in order to impart a P type conductivity to silicon and adjust a conductivity thereof to a prescribed value, whereas on the other hand, boron attached on a silicon wafer coming from an atmosphere in which the silicon wafer is handled, and an apparatus, materials and others used in processing steps acts as an extrinsic factor to disturb electric characteristics of the wafer; therefore, it is required to prevent contamination due to boron from the atmosphere, apparatus, materials and others as much as possible.
Assume that boron is attached on a silicon wafer surface at a concentration of 1×1010 atoms/cm2 and diffused into the interior from the surface (on the order of 1 μm in depth), this diffusion corresponds to several ohm·cm in electric resistivity of the surface layer; therefore, electric characteristics of the surface layer, in which devices are fabricated, would be considerably affected with a variation if a bulk electric resistivity of the wafer is 1 ohm·cm or higher.
Furthermore, in cases of a wafer with a PBS film and an epitaxial wafer with a PBS film, boron attached on a wafer surface on which a PBS film is to be formed diffuses mainly into the PBS film in heat treatment of a PBS film forming process and an epitaxial layer growing process following the PBS film forming process. Moreover, when boron is attached on a substrate surface on which an epitaxial layer is to be grown, the element diffuses into the interior of the substrate and the epitaxial layer from the interface therebetween by heating during growth of the layer. This may cause a trouble that a device fabricated using an epitaxial wafer whose substrate has an electric resistivity of several ohm·cm or higher cannot achieve prescribed characteristics thereof.
Furthermore, when an epitaxial layer is grown in manufacture of an epitaxial wafer, a PBS film on the back surface results in a non-uniform film thickness and surface roughening in a wafer periphery thereof due to etching with an atmospheric constituent gas in a growth chamber. For the purpose to prevent such inconveniences, a silicon oxide film (CVD silicon oxide film) has been further formed on the PBS film by means of a CVD (Chemical Vapor Deposition) method, but if boron contamination occurs in this CVD process similar to the above described as well, attached boron diffuses mainly into the PBS film by temperature increase in the CVD process and the following epitaxial layer growing process, which exerts an adverse influence on the device characteristics as in the above described.
Concrete description will be given of the prior art problem described above with respect to a case of an epitaxial wafer with a PBS film as an example. FIG. 10 is a descriptive diagram showing a prior art manufacturing process for an epitaxial wafer with a PBS film, wherein (a) shows a silicon wafer W, (b) a state after a polycrystal silicon layer 62 is formed on the silicon wafer W, (c) a state after a CVD silicon oxide film 64 is further formed on the polycrystal silicon layer (PBS film) 62 on the back surface of the silicon wafer of (b) in order to prevent the above described etching in the following epitaxial layer growing step, (d) a state after the polycrystal silicon layer 62 on the front surface of the silicon wafer is removed by mirror polishing, and (e) a state after an epitaxial layer 66 is formed on the front surface of the silicon wafer following the state of (d).
In manufacture of the epitaxial wafer with a PBS film on a back surface thereof according to a prior art, boron is attached at the interface 70 between the silicon wafer W and the polycrystal silicon layer (PBS film) 62, which causes contamination (FIG. 10(b)). Then, boron contamination occurs at the interface 72 between the polycrystal silicon layer 62 and the CVD film 64 (FIG. 10(c)). After the polycrystal silicon layer 62 on the silicon wafer W is removed (FIG. 10(d)), an epitaxial layer growing step (high temperature heat treatment) is applied and thereby boron present at the interfaces 70 and 72 diffuses into the PBS film 62 and further contaminates the PBS film in the entirety thereof to produce the boron-contaminated PBS film 62a (FIG. 10(e)).
On the other hand, in a clean room (containing a cleaning apparatus, a wafer storage box, a film growing apparatus and so on) or the like where prior art treatments of a semiconductor wafer, for example cleaning, transportation, storage, film growth and others are performed, there has been no knowledge of how much boron included in an atmosphere with which a wafer is in contact is attached onto a surface of the wafer. Accordingly, there has been absolutely no knowledge of a value or less at which if a boron concentration in the atmosphere with which the wafer is in contact is controlled, a boron amount attached on the wafer from the atmosphere can be restricted to a value or less at which neither of device characteristics are adversely affected in any way, which has made effective prevention of boron contamination in manufacture of a high quality wafer difficult in a practical aspect. That is, since it takes much of time for measurement of a boron amount attached on a wafer in process, a result of the measurement cannot be immediately fed back on the spot to the apparatus side; therefore, it has been impossible to perform on-line control for prevention of boron contamination. Hence, it has been impossible to manufacture a wafer stabilized in quality as far as control for prevention of boron contamination is effected based on measurement of a boron amount on a wafer.
Besides, air filters used in a clean room (containing a cleaning apparatus, a wafer storage box, a film growth apparatus and so on) or the like where prior art treatment of a semiconductor wafer, for example cleaning, transportation, storage, film growth and others is performed are generally made from glass fibers; therefore, it is known that if a corrosive gas such as hydrogen fluoride is present in an atmosphere when a wafer is treated, the gas is put into contact with the glass fibers to dissolve boron therein out into the atmosphere.
As air conditioning facilities for a clean room, a construction as shown in FIG. 11 has been adopted. In FIG. 11, 12 indicates air conditioning facilities constituting a clean room 14 and in the clean room 14, provided are various kinds of wafer treatment rooms such as, in a case shown in the figure, a CVD treatment room 16, a CVD furnace room 18, a PBS film forming room 20 and a PBS furnace room 22. One or more air inlet ports are provided in each of the rooms 16, 18, 20 and 22, and air filters 16a, 16b, 18a, 20a, 20b and 22a are equipped to the respective air inlet ports.
Various kinds of wafer treatment apparatuses are installed in the respective wafer treatment rooms. For example, in the CVD treatment room 16 disposed are a CVD treatment precleaner 24, a dryer 26 and a storage clean booth 28. In the CVD furnace room 18, a CVD furnace 30 is disposed. In the PBS film forming room 20, disposed are a PBS film formation precleaner 32, a dryer 34 and a storage clean booth 36. In the PBS furnace room 22, a PBS apparatus 38 is disposed. Air filters 24a, 26a, 28a, 30a, 32a, 34a, 36a and 38a are mounted to air inlet ports of the apparatuses 24, 26, 28, 30, 32, 34, 36 and 38. A numerical mark 39 indicates an air outlet space disposed on the downstream side of the clean room 14. The air outlet space 39 is connected to a recovery pipe 43.
An outdoor air cleaner 40, an impurity removal apparatus 42 and an air conditioner 44 are in series disposed along a direction from upstream to downstream on the upstream side of the clean room 14. The outdoor air cleaner 40 and the impurity removal apparatus 42 are connected to each other by a first air pipe 46 and the impurity removal apparatus 42 and the air conditioner 44 are connected to each other by a second air pipe 48. A third air pipe 50 connects the air conditioner 44 with each of the air filters 16a, 16b, 18a, 20a, 20b and 22a mounted at the air inlet ports of the respective rooms of the clean room 14.
The outdoor air cleaner 40 includes a roll filter 99 and a medium-performance filter 52, both of which are disposed at the air inlet side, and a fan 54 at the air outlet side. The impurity removal apparatus 42 is constituted of a chemical filter for removing NOx and SOx. The air conditioner 44 includes an air inlet fan 56 and a prefilter 98 in this order and subsequent to this a medium-performance filter 58 at the air outlet side.
In the prior art clean room air conditioning facilities 12 having the above described construction, the outside air passes through the outdoor air cleaner 40, the impurity removal apparatus 42 and the air conditioner 44 and is then supplied into the interior of the CVD treatment room 16, the CVD furnace room 18, the PBS film forming room 20 and the PBS furnace room 22 through the respective air filters 16a, 16b, 18a, 20a, 20b and 22a. While, generally speaking, CVD treatment is in that one or more kinds of chemical compound gases or single element gases including element or elements constituting a thin film material are supplied onto a wafer to form a desired thin film on a wafer surface by a chemical reaction in a vapor phase or on the surface, for example, silicon oxide is vapor deposited on a wafer back-surface on which a PBS film is formed in advance, which can be adopted as treatment to prevent etching of a PBS film and auto-doping from occurring.
Air introduced into the rooms is further introduced into the cleaner 24 through the air filter 24a, into the dryer 26 through the air filter 26a, into the storage clean booth 28 through the air filter 28a, into the CVD apparatus 30 through the air filter 30a, into the PBS precleaner 32 through the air filter 32a, into the dryer 34 through the air filter 34a, into the storage clean booth 36 through the air filter 36a, and into the PBS apparatus 38 through the air filter 38a; and finally discharged into the air outlet space 39, wherein the air is partly discharged directly from the cleaners 24 and 32, and all the air provided is discharged directly from the CVD apparatus 30. The air discharged into the air outlet space 39 amounts to about 80% of the total supply and is recycled after being returned to the second air pipe 48 through the recovery pipe 43.
In such a prior art clean room air conditioning facilities, ULPA filters (for example, Nippon Muki Co., Ltd. NMO-320) are generally employed as the air filters 16a, 16b, 18a, 20a, 20b, 22a, 24a, 26a, 28a, 30a, 32a, 34a, 36a and 38a. This ULPA filter has no function to remove boron, but on the contrary, has a risk to release boron either. Furthermore, the medium-performance filters 52 and 58 (for example, Nippon Muki Co., Ltd. ASTC-56-95) has no function to remove boron either, but on the contrary, has a risk to release boron. Hence, control of a boron concentration in an atmosphere of a clean room absolutely cannot be effected with prior art air conditioning facilities in which such filters are incorporated.
Moreover, while a boron-less filter (an air filter from which no boron is released) and a boron adsorbing filter (a filter which adsorbs boron therein) are known, it is a current state of the art that no proposal on a good method has been made in which these filters are efficiently utilized to suppress an attached amount of boron on a wafer to a prescribed value or less. There is available a reference in which an adverse influence of boron in an atmosphere of a clean room on manufacture of a bonding-type substrate is discussed, in which while only difficulty in realization of removing facilities in a practical use is pointed out (for example, JP-2723787), discussion is presented about neither effective removal of boron nor control of a boron content in an atmosphere of a clean room within a prescribed range.
In order to solve the above described problems associated with a wafer and an epitaxial wafer, the inventors have quantitatively determined a relationship between a boron concentration in an atmosphere with which the wafer and the epitaxial wafer are in contact and a boron amount attached onto a surface of each of the wafers placed in the atmosphere and have further empirically made clear an influence of an attached boron amount on boron concentration distributions in the bulks of various products, and based on such knowledge and findings, have finally succeeded in restricting boron contamination coming from the environment and besides manufacturing the wafer and the epitaxial wafer causing no adverse influence on device characteristics therefrom in a stable manner.
It is accordingly a first object of the invention to provide a silicon wafer and an epitaxial wafer, both stabilized in quality, which exert no adverse influence on device characteristics, manufactured by restricting a boron contamination from the environment; and effective manufacturing processes therefor.
In order to solve the above described problems of prior art clean room air conditioning facilities, the inventors further continued to conduct serious studies on a relationship between a boron concentration in an atmosphere and an attached boron concentration on a wafer, by performing experiments in which the air in a clean room (containing a cleaner, a wafer storage box, a film growing apparatus and so on) was dissolved into pure water in an impinger and the pure water after the dissolution was analyzed by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) to measure a boron concentration in the atmosphere, with the result that the inventors have succeeded in not only discovering that there is a relationship between a boron concentration in an atmosphere and an attached boron concentration on a wafer, but also suppressing the boron concentration in an atmosphere of a clean room to a prescribed concentration or less.
It is a second object of the invention to provide an atmosphere control apparatus, a clean room and clean room air conditioning facilities capable of preventing boron contamination on a wafer at low cost.